1. Field of the Invention
The present invention relates to an addition-curable silicone rubber composition and a cured product thereof, and relates particularly to a curable silicone rubber composition that produces a cured product with a superior refractive index, favorable rubber-like properties and strength characteristics, almost no surface tackiness, and a particularly superior resistance to thermal shock, as well as a cured product produced from such a composition.
2. Description of the Prior Art
Silicone rubber compositions exhibit excellent properties of weather resistance and heat resistance and the like, and form cured products with superior rubber-like properties such as hardness and elongation, and are consequently used in a wide variety of applications, but because they also exhibit surface tackiness, dust adhesion becomes a problem when such silicone rubber compositions are used as coating agents or the like for electrical or electronic components.
In the case of silicone varnishes that resolve this problem of surface tackiness, the occurrence of cracking becomes a problem. Furthermore, reduced hardness resins produced by adding a silicone oil to a silicone varnish are extremely brittle, and suffer from problems of reliability. A silicone rubber composition capable of forming a cured product with no surface tackiness and excellent crack resistance has been keenly sought for use as packaging for electrical and electronic components and the like. Furthermore, in the case of optical components and optical semiconductor elements and the like, increases in the refractive index are seen as a very desirable optical property.
In addition-curable silicone rubber compositions, the addition of a resin-like organopolysiloxane to improve the strength of the cured product is a commonly known technique. However, even in those cases where the strength of a cured product is raised by adding a resin-like organopolysiloxane, surface tackiness and dust adhesion remain problems.
As a result of previous intensive investigation, we have previously discovered an organopolysiloxane composition and a cured product thereof that are capable of resolving the problems outlined above. The cured product of this organopolysiloxane composition exhibits a dramatically higher resistance to thermal shock than high hardness silicone resins. However, in the case of packages of optical components or the like comprising two or more substances with different coefficients of linear expansion, although the composition exhibits reflow temperature resistance (260° C. and 275° C.), the composition may not exhibit satisfactory resistance to thermal shock (resistance to heat cycling).
Accordingly, as a result of further intensive investigation, we discovered that by reducing the quantity of low molecular weight products incorporated within the organopolysiloxane, a resin with even superior resistance to thermal shock could be obtained.
Patent reference 1 discloses a sealing agent for an optical material containing a polyorganosiloxane as the base polymer, wherein the weight loss upon heating at 200° C. for one hour is not more than 1.5% by weight, but this sealing agent represents a different concept from the base polymer of the present invention, and furthermore, the above patent reference includes no disclosure regarding resolving the cracks that are generated under the thermal shock of heat cycling.
[Patent Reference 1] JP 2005-307015A